1. Field of the Invention
The present invention relates generally to particle size analysis apparatus and more specifically to an improved mixing reservoir used in an automated recirculating particle size analysis system.
2. Discussion of the Related Art
Automated recirculating particle size analysis systems are used to prepare a slurry by suspending the particles to be measured in a liquid and to continually stir the slurry to provide a homogenous suspension. The slurry is then continuously recirculated through an analyzer during analysis. Typically such recirculator systems utilize a mixing chamber or reservoir in which a stirring impeller is disposed to thoroughly mix the particles. The slurry with its suspended particles is pumped from the mixing reservoir to the analyzer and then returned to the reservoir. The analysis apparatus conventionally includes a sample cell where an included analysis apparatus measures the particle distribution content of the slurry. One such automated recirculating particle size analysis system is disclosed by U.S. Pat. No. 5,439,288 to Jeffrey G. Hoffman et al., and which is now assigned to the same assignee as the present invention.
It is of vital importance in such systems that the distribution of the particles in the slurry in the sample cell be representative of the entire statistical population to ensure valid data collection for analysis.
In apparatuses that measure particle size in the micron particle size ranges of less than 100 microns, even the densest materials disperse uniformly throughout the fluid and provide a uniform slurry. However, particles in the size range between 100 to 1000+ microns have movement that tends to become more independent of the fluid. When the slurry is reintroduced to the mixing reservoir for recirculation, the large dense particles tend to fall straight to the exit port. Since large particles are under-represented in the mixing tank, they are over-represented in the sample cell of the measuring apparatus, therefore, developing a non-uniform distribution of particles within the recirculating apparatus. This non-uniform distribution does not accurately represent the statistical population of particles in the slurry.
Prior art methods for keeping large particles in suspension in the reservoir tank included the use of high-speed impellers for shifting and directing the particles away from the reservoir outlet and/or directing the return flow against a deflecting surface so as to deflect the returning particles into the larger volume of the reservoir. Such prior art devices have been found to have drawbacks in that the impeller typically introduces excess turbulence and bubbles within the reservoir due to the speed of the impeller and its propensity to cause cavitation in the fluid. Additionally, the larger delicate particles may be broken into smaller pieces when battered against hard surfaces during injection onto deflecting surfaces and, therefore, not correctly represent the particle size distribution found in the manufacturing process.
It is, therefore, an object of the present invention to provide an improved mixing reservoir that maintains a uniform distribution of large dense particles within a slurry.
It is also an object of the present invention to provide an improved mixing reservoir that maintains a uniform distribution of particles in suspension without the aid of mechanical devices such as motor driven impellers or other high vortex inducing devices.
In carrying out the objects of the invention, there is provided an improved mixing reservoir for supplying particles suspended in a liquid, to form a slurry, for delivery and recirculation within a recirculating system. The mixing reservoir includes an outlet port at its lowest point connected to a recirculation pump. The pump is arranged to draw the slurry from the mixing chamber and to cause a stream of said slurry to flow through the recirculating system. The improved mixing reservoir includes a plurality of inlet ports located on the mixing reservoir with each inlet port connected to the recirculating system and the stream of slurry, whereby the particles contained in the slurry are retained in suspension by the resulting chaotic motion of the colliding streams of slurry as they are returned to the mixing reservoir.